Silicone rubber compositions which cure at room temperature by a condensation reaction to become rubber-like elastomers are generally referred to as "RTV (Room Temperature Vulcanizing) silicone rubbers". As a result of the ease in carrying out the curing reaction and various other features (i.e., the cured products have excellent heat resistance and cold resistance, do not easily undergo changes in physical properties with the change of temperature, and further do not suffer from substantial deterioration of physical properties from exposure to ozone and ultraviolet rays), the RTV silicone rubbers have been widely employed in various fields and applications including sealants for construction.
RTV silicone rubbers are mainly composed of a polyorganosiloxane as a base material, a crosslinking agent and a filler. The rubbers are classified into various categories according to the type of crosslinking reaction which is based on the crosslinking agent used and which results in different condensation reaction by-products.
The dealcoholization-type RTV silicone rubbers produce an alcohol by-product which is free from odors and corrosive action and further does not easily generate bubbles or undergo cracking on curing. These types of rubbers have been employed as sealed insulating glass sealant materials for in-plant installation types, replacing the previous on-spot installation types, and have been under study as adhesive sealant materials for general industrial uses. However, these rubbers have drawbacks in that their adhesive properties and durability are poor, that is, the strength and modulus are low, and in particular, the strength and modulus are greatly lowered and the sealing properties deteriorated under high temperature and high humidity circumstances.
Dry silica, wet silica, diatomaceous earth, ground quartz, calcium carbonate, alumina etc. are generally employed as fillers in RTV silicone compositions. The silicas are the most frequently employed fillers, and RTV silicone rubbers utilizing these can possess high strength. However the amount of filler added is greatly restricted because of the thickening properties generated when mixed with an organopolysiloxane component. The use of ground quartz and alumina does not result in high thickening properties, but the obtained strength is not as high as expected.
Further, the calcium carbonate is classified as either ground limestone or light limestone depending on whether it is natural or synthetic, and the latter is further classified into colloidal calcium carbonate and light calcium carbonate. These come in various grades according to the particle size and whether and how surface treatment has been conducted, and since the thickening properties and strength vary depending on the grade used as the filler, the calcium carbonates are appropriately selected for use according to the purpose and application.
The use of calcium carbonate as the filler in a dealcoholization-type RTV silicone rubber is known in the art. For example, the retention of the surface light release properties of a tonor-fixed roll of a copying machine is improved by combining calcium carbonate having average particle size of 10 .mu.m or less with powdered iron oxide and powdered titanium oxide (Japanese Patent Application Laid-open No. 2439/1977); an adhesion-improved room temperature vulcanizing organopolysiloxane composition for sealing which can control the flowability, and is excellent in deep part curing properties and the cured product of which has such physical properties as high elongation and low modulus is obtained by using calcium carbonate substantially free from surface treatment and having an average particle diameter of 0.3 .mu.m or less and a specific surface area as measured by an air transmission method of 3.0 m.sup.2 /g or more (Japanese Patent Publication No. 18454/1980); and an example which imparts especially excellent heat resistance and greatly improves the change in the physical properties and the heat loss by using light calcium carbonate having a specific surface area as measured by a BET method of 1-30 m.sup.2 /g and an average particle diameter of 0.3-10 .mu.m and of a spindle shape and the surface of which has been treated with an organosilicon compound (Japanese Patent Publication No. 23940/1986).
However, although the use of calcium carbonate, when incorporated in RTV silicone rubbers, resulted in improved workability, surface release properties, adhesion etc., the RTV rubbers had low modulus and high extensibility etc., and rubbers having high strength, high modulus and low extensibility could not be obtained.
It is desirable to provide dealcoholization-type RTV silicone rubbers incorporating calcium carbonate wherein the rubbers are useful as sealants for sealed insulating glass and possess high strength, high modulus, low extensibility, adhesion and durability.